In a field of a wireless communication, the technologies for realizing a stable data communication by dynamically changing a communication rate depending on a communication environment such as a radio signal propagation condition have been disclosed. For example, Japanese Patent Application Laid-Open No. 2004-297118 discloses the technology which decreases a data error rate and decreases a delay in a data transmission by reducing the communication rate when a communication state is deteriorated. Japanese Patent Application Laid-Open No. 2005-294997 discloses the technology which increases the communication rate gradually when starting a packet transmission to a mobile station, and decreases the communication rate gradually when ending the packet transmission. The object of this is to prevent the degradation of the communication quality in other mobile stations, which are caused by an interference power, by suppressing a sudden change of the electric power generated by the packet transmission.
In a wireless LAN (Local Area Network), there are many factors which adversely affect the communication quality, such as not only obstacles existing in a radio signal propagation path but also communication interference by many access points and the like. In the wireless LAN, the rate adaptation function is incorporated, i.e., a plurality of communication rates are provided and the communication rates can be dynamically changed according to the communication environment.
Specifically, IEEE (The Institute of Electric and Electronics Engineers, Inc.) standardizes the standard for the wireless LAN such as IEEE802.11a, IEEE802.11b, IEEE802.11g and the like. For example, the IEEE802.11a standard specifies eight kinds of communication rates, that are 6 Mbps (Mega bit per second), 9 Mbps, 12 Mbps, 18 Mbps, 24 Mbps, 36 Mbps, 48 Mbps and 54 Mbps, as the communication rate available in a physical layer.
The above-mentioned communication rate (6 Mbps, 9 Mbps, 12 Mbps, 18 Mbps, 24 Mbps, 36 Mbps, 48 Mbps or 54 Mbps) is treated as an indicated rate which is only available as an instantaneous value of rate and communication is not always performed at these rates. An actual communication rate (effective rate) is lower than these rates caused by, for example, a process for an application program, an existence of overhead of a data-transfer protocol, and a waiting time process for transmitting and receiving a data frame that is peculiar to the wireless LAN, such as a DIFS (Distributed Coordination Function Inter-frame Space) and a SIFS (Short Inter-frame Space). Moreover, when a communication radio signal power is weakened due to a long hop between the wireless LAN equipments, obstacles existing in the radio signal propagation path, radio signal interference from other access points or the like, the effective rate becomes lower.
A wireless LAN system, which conforms to the IEEE802.11a standard, selects one of the eight available communication rates for a data transmission. When it is judged that the communication quality is improved and it becomes better communication state, the wireless LAN system controls to increase the communication rate by one step. On the other hand, when it is judged that the communication quality is deteriorated and it becomes worse communication state, the wireless LAN system controls to decrease the communication rate by one step. Thus, the wireless LAN system conforming to the IEEE802.11a standard optimizes the transmission efficiency by the rate adaptation function of the best effort type which changes the communication rate according to the communication state.